1. Field of the Invention
The present invention relates to a novel method for converting a tone of a picture upon producing a printed picture in a halftone from a photographic are final picture in a continuous tone.
Particularly, the present invention provides a method for converting a tone of a picture, which method is effective in the event a printed picture having appropriate tonal characteristics is intended to be produced from an original picture as a photographic original which does not have a wide density range from H part (the brightest part) to S part (the darkest part), namely said original picture not having both of the H and S parts or either one of them, or in the event that a printed picture having entirely appropriate tonal characteristics is intended to be produced by totalizing sectional (stepwise) information inputted from a sensor when only such the divisional (stepwise) picture information is available due to characteristics of the sensor.
2. Prior Art
As well known, what becomes a crux a production process of a printed picture from a photographic original picture is a color-separation technique.
Although a scanner (including monochrome- and color-scanner, hereinafter the same), which is manufactured introduced modern technologies such as electronics, computer, and the like, is used as a working tool in the color-separation technique nowadays, the color-separation technique is essentially based on experiences and intuition recklessly, it still thus remaining in a non-scientific and irrational stage. Such circumstances are bespoken by the fact that it is still taken up as a subject in the controversy whether the color-separation work is a technology or an art.
Giving a further explanation about this, the conventional color-separation technique has been developed on the basis of a color photograph processing technique or a photographic masking technique. Namely, in the color-separation technique, a fundamental way of thought of formation of a printed picture excessively inclines toward the processing technique of photographic pictures so as to fail to understand, study and analyze the essence of printed pictures themselves and the production process of a printed picture. Because only the color-separation technique established from a viewpoint of the photographic processing technique is considered to be an object of studies and developments in the conventional color-separation technique so that conversion of a picture tone, which is positioned in the first step in the formation of a printed picture and gives a decisive effect to the quality of a printed picture, a following relationship between the tone and the color-tone of a printed picture, and techniques of adjustment, correction, change, etc. of color are placed outside studies and developments. As a result, these techniques still remain in a state based on experiences and intuition of man, which can be considered to be a fundamental fault.
A scanner is used nowadays in the color-separation work as a modern working device, but setting of a tonal conversion curve (referred to as "a tone curve" in a monochromatic scanner, "a color-separation curve" or "color-separating characteristic curve" in a color scanner, or the like, hereinafter the same), which becomes a core of the working operation, is carried out, by selecting a suitable one (however no assurance if it is satisfactory or not) out of several tonal conversion curves which are made on the basis of experiences of the scanner manufacturer and stored in the storing system of the scanner. Or it is carried out by calling out, when necessary, a tonal conversion curve stored in the storing system of the scanner, which has been made on the basis of experiences and intuition of each scanner user, and considered to be the most suitable to his own work conditions. Such present circumstances are attributable to that no rational theory is given to a work of tone adjustment of a picture, two essential works (adjustment of tone and adjustment of color-tone) in the color-separation process being treated by a color scanner at present.
This is a good instance showing a fact that the conventional color-separation technique is essentially based on experiences and intuition of man although used therein a scanner.
Likewise, a high-level mechanism is adopted in the electronic plate-making technical system such as monochromatic scanner, color-scanner, or the like, but a tonal conversion curve which becomes a core of the working operation by a scanner is made unscientifically and irrationally. Scientific and rational formation of a tonal conversion curve which becomes a core in the working operation of a scanner is, in fact, an essential prerequisite to scientifically and rationally carry out conversion of a picture tone, simultaneous maintenance of a tone and a color-tone, and the like.
The present inventors have solved and overcome these disadvantages and limitations of the conventional color-separation technique mentioned above, and made a various proposals to establish a scientific and rational color-separation technique.
That is, the present inventors proposed a number of techniques, a technique for scientifically setting a tonal conversion curve, which becomes a crux of the work upon forming a printed picture in a halftone, and conducting tonal conversion of a picture by utilizing such the tonal conversion curve set in the scientific manner (Japanese Patent Application No. 62-148912, U.S. Pat. No. 4,811,108), a technique for plate-making applied the above tonal conversion technique for a picture (Japanese Patent Application No. 62-165231, U.S. Pat. No. 4,833,546), a technique for controlling a halftone dot area percent, which is applied the above tonal conversion technique for a picture (Japanese Patent Application No. 62-198302), and on.
The present inventors proposed graded-up techniques of these for setting a tonal conversion curve, which allow them to comply with not only an original picture having a standard picture quality but also a non-standard original such as over-, under-exposed, or the like (Japanese Patent Application Nos. 63-114599, 63-207326), as well.
Additionally, the present inventors further proposed another techniques (Japanese Patent Application No. 1-135825) in order to shift the conventional technical system, in which density information values are utilized as picture information values of an original upon setting a tonal conversion curve, into a novel technical system, in which picture information values relating to light intensity (light exposure) obtained from a subject (real scene) forming a base of the original picture are utilized.
The said techniques (Japanese Patent Application No. 1-135825 and the like) being adopted light intensity information values therein to set a tonal conversion curve are quite unique, whereby a printed picture having an appropriate tone became available without effected by the quality of an original picture (over- or under-exposed original, or original in a high- or low-key which is said to be difficult to convert its tone, or color-fogged original). Further, since a tonal conversion curve (a curve defining a relationship between light intensity information values obtained from density information values and dot area % values) set according to these techniques defines tonal characteristics (a state of dot arrangement) of a finally produced printed picture, it was found that these techniques have such a superior performance that an observation of a tonal conversion curve makes it possible to skip a proofing, at least a printer's proofing. On the other hand, the conventional tonal conversion curve which is defined on the basis of a relationship between density information values and dot area % values has a complicated shape so that it is difficult to tell the tonal characteristics of a finally obtained printed picture only by observing the curve. Consequently, it is necessary to conduct a proofing to evaluate the adequacy of a tonal conversion.
It is an object of the present invention to provide a method for converting a tone of a picture to produce a printed picture having appropriate tonal characteristics from an original picture devoid of both of its H and S part or either one of them, while being employed therein a technique for setting: a tonal conversion curve on the basis of light intensity information values, which technique has been proposed by the present inventors. There is a strong demand for development of a rational tonal conversion method, as the conventional technique completely depending upon experiences and intuition of man.
There is provided a method for converting a tone of a picture upon producing a printed picture in a halftone from a photographic original picture in a continuous tone, in which a density information value (D.sub.n) of each pixel of the photographic original picture into a light intensity information value (X.sub.n) by employing a photographic density characteristic curve of the photographic original, which is expressed in an D-X orthogonal coordinate system of which D axis represents density information values and X axis represents light intensity information values, and determining a tonal conversion intensity value (y.sub.n) from said X.sub.n value by way of a tonal conversion formula (1), which method comprising the steps of:
(1) defining a desired .gamma..sub.o value as a .gamma..sub.n value in the tonal conversion formula (1) to define a reference tonal conversion curve (y.sub.o) in order to obtain a picture with a desired tone from a reference photographic original picture, which becomes a reference, having values ranging from a H (highlight) part density value (D.sub.Ho) to an S (shadow) part density value (D.sub.So), said reference tonat conversion curve (y.sub.o) being defined from relationships of the tonal conversion intensity values (y.sub.n) which are determined from light intensity information values (X.sub.Ho -X.sub.So) converted from the corresponding density values (D.sub.Ho -D.sub.So) by conducting the tonal conversion formula (1) by employment of the .gamma..sub.o value therein;
(2) determining an .gamma..sub.n value in the tonal conversion formula (1) by conducting formula (2): EQU .gamma..sub.n =(.gamma..sub.o).times.[(X.sub.Hn -X.sub.Sn)/(X.sub.Ho =S.sub.So)] (2)
in order to adopt an individual tonal conversion curve (y.sub.n) of an individual photographic original picture having: a density range from (D.sub.Hn) to (D.sub.Sn) falling within the range from the H part density value (D.sub.Ho) to the S part density value to the reference tonal conversion curve (y.sub.o) in a range from (X.sub.Hn) to (X.sub.Sn), said individual tonal conversion curve (y.sub.n) being defined from a relationship of the tonal conversion intensity values (y.sub.n) which are determined from light intensity information values (X.sub.Hn -X.sub.Sn) converted from the corresponding density values (D.sub.Hn -D.sub.Sn) by conducting the tonal conversion formula (1) by employment of the .gamma..sub.n value therein; and
(3) conducting a tonal conversion such as to determine the tonal conversion intensity value (y.sub.n) from the light intensity information value (X.sub.Hn -X.sub.Sn) corresponding to the density information value (D.sub.Hn -D.sub.Sn) of the individual photographic original picture by making use of the .gamma..sub.n value and the tonal conversion formula (1): ##EQU1## where
n: becoming n when applied to the individual photographic original picture, while becoming 0 when applied to the reference photographic original picture;
X.sub.n : showing a basic light intensity information value (X.sub.n =X.sub.n '-X.sub.Hn) determined by subtracting the light intensity information value (X.sub.Hn) of a corresponding pixel, which value (X.sub.Hn) is determined from the density information value (D.sub.Hn) of the brightest (H) part on the photographic original picture via the photographic density characteristic curve, from the light intensity information value (X.sub.n ') of a corresponding pixel from the density information value (D.sub.n) of an arbitrary pixel on the photographic original picture by making use of the photographic density characteristic curve;
X.sub.Sn, X.sub.Hn : light intensity information values (X.sub.Sn, X.sub.Hn) of corresponding pixels determined from the density information values (D.sub.Sn, D.sub.Hn) of the darkest (S) and brightest (H) parts on the photographic original picture via the photographic density characteristic curve, respectively;
y.sub.n : (of the reference or individual) a tonal intensity value preset to a pixel on a reproduced picture corresponding to an arbitrary pixel on the photographic original picture;
y.sub.Hn : (of the reference or individual) a tonal intensity value preset to the brightest (H) part on the photographic original picture;
y.sub.Sn : (of the reference or individual) tonal intensity value preset to the darkest (S) part on the photographic original picture;
.alpha.: a surface reflectance of a printed paper used to express a printed picture;
.beta.: a value determined by .beta.=10.sup.-.gamma.n ; and
.gamma..sub.n : an arbitrary coefficient.
An advantage of the tonal conversion method of a picture of this invention conducted upon producing a halftone printed picture from a photographic original picture in a continuous tone is that the method makes it possible to quite rationally convert the tone of the photographic picture not having a wide range spreading from its H part (brightest part) to S part (darkest part), namely devoid of both of the H and S parts or either one of them.
Another advantage of the tonal conversion method of a picture according to this invention is that the method also makes it possible to produce a printed picture having appropriate tonal characteristics, when looking throughout the picture, by unifying sectional (stepwise) input information of a picture, when applied to a system for obtaining information which is suitable rather to sectional (stepwise) picture information due to the characteristics of an input sensor of the system.